1. Field of the Invention
The present invention relates to a variable capacitance device that provides a variable capacitor by using a Micro-Electro-Mechanical Systems (MEMS) device driven by electrostatic attraction.
2. Description of the Related Art
In the past, a variable capacitance device using an electrostatically driven MEMS has been used in some cases (see Japanese Unexamined Patent Application Publication No. 09-17300 and Japanese Patent No. 3538109).
FIGS. 1A-1C are diagrams for explaining a configuration example of an existing variable capacitance device.
A variable capacitance device 101 includes movable plates 102 and 103. The movable plates 102 and 103 are each made of a conductive material, have a doubly supported beam structure, and are disposed next to each other with a gap therebetween. The movable plate 103 has a convex surface facing the movable plate 102 and is provided with a dielectric layer 104. When a DC voltage is applied between the movable plates 102 and 103, electrostatic attraction attracting the movable plates 102 and 103 to each other is generated in the movable plates 102 and 103, and the gap between the movable plates 102 and 103 is reduced. When a convex tip region of the movable plate 103 comes into contact with the dielectric layer 104, the capacitance of the variable capacitance device 101 is substantially increased. If the DC voltage is changed, the contact area of the movable plate 103 with the dielectric layer 104 changes, and the capacitance changes in accordance with the contact area.
In the existing variable capacitance device, a DC voltage and an RF signal are applied to a common electrode. Therefore, it is necessary to separate a DC component and an AC component from each other by inserting a resistor and a capacitor in a DC voltage line and an RF signal line. The insertion of a resistor and a capacitor leads to the possibility of an increase in insertion loss and so forth. It is therefore conceivable to structurally separate the DC voltage line and the RF signal line from each other to thereby electrically separate the DC component and the AC component from each other without using a resistor and a capacitor. Also in this case, however, if an electrode connected to the DC voltage line and an electrode connected to the RF signal line are formed in the same dielectric material, capacitive coupling occurs between the two electrodes. As a result, it is difficult to completely electrically separate the DC component and the AC component from each other. To prevent property degradation of the RF signal, it is necessary to set the coupling capacitance to, for example, 0.1 pF or less.